(NewsNation Now) — The Centers for Disease Control and Prevention on Thursday increased its COVID-19 travel health notice to the highest level, telling all Americans to avoid them regardless of their vaccination status. It reflects a rise in cases onboard cruise ships since the emergence of the omicron variant.

The CDC now is warning against cruise travel regardless of vaccination status in light of level four, or “very high” levels of COVID-19. The travel health notice level previously was at a three.

As of Wednesday, the agency said 88 vessels were either under investigation or observation, although the CDC didn’t specify how many cases have been reported.

While none of the ships appeared to have enough cases to overwhelm medical resources on board and require a return to port, some have been denied entry at foreign ports.

Anyone who does travel on a cruise shop should be fully vaccinated beforehand and receive a COVID-19 vaccine booster dose if they are eligible, according to the CDC.

Cruise travelers also should be tested for COVID-19 one to three days before their trip and three to five days upon return, regardless of their vaccination status or symptoms. Along with testing, passengers who are not fully vaccinated should self-quarantine for a fully five days after cruise travel, according to the CDC.

Travelers keep their noses and mouths covered with a mask while they are in shared spaces and individual cruise lines may require travelers, passengers and crew to wear masks while they are on board ships.

The advisement comes on the heels of recent changes to the CDC’s quarantine guidelines as COVID-19 cases continue to rise.

Under the new guidance, people with COVID-19 should isolate for 5 days and if they are asymptomatic or their symptoms are resolving. They should continue wearing a mask while around others for the following five days to minimize the risk of infection, according to the CDC.

The change was motivated by science that showed the majority of COVID-19 transmission happens early, generally in first one to two days before the onset of symptoms and the two to three days after, according to the CDC.

On NewsNation’s Morning in America on Thursday, Dr. Anthony Fauci said the adjustments were designed to keep people at work.

“The CDC made a decision to balance what’s good for public health, at the same time keeping the society running,” Fauci said.

The U.S. recently surpassed 50 million COVID-19 cases and 800,000 deaths since the start of the pandemic, according to the CDC.

This week also marks the first anniversary of the first COVID-19 vaccination in the country.